Hydrogen fatigue resistant ferritic steel and manufacturing method thereof

ABSTRACT

A ferritic steel having tensile properties and fatigue properties capable of withstanding use in a hydrogen environment and a method of manufacture thereof are provided. By adding one or more element selected from among vanadium (V), titanium (Ti) and niobium (Nb) so that the steel includes, together with at least ferrite grains in the structure, a carbide or carbides of one or more element selected from among V, Ti and Nb, the reduction of area and the fatigue crack propagation rate of the ferritic steel in a hydrogen environment are improved. The advantages of the invention were confirmed in cases where the ferrite grains are small grains of 1 μm or less in size, and in cases where the ferrite grains are coarse grains from several micrometers to 20 μm in size, and moreover in cases where the ferrite grains are coarse grains from several micrometers to 60 μm in size.

TECHNICAL FIELD

The present invention relates to metal materials used in a hydrogenenvironment. More particularly, the invention relates to ferritic steelshaving excellent tensile properties and fatigue properties in a hydrogenenvironment, and to a method of manufacture thereof.

BACKGROUND ART

From a global environmental standpoint, large expectations are beingplaced on having hydrogen energy systems such as fuel cell vehicles anda hydrogen energy infrastructure of hydrogen stations and the likebecome a reality. However, metal materials exposed to hydrogen in ahydrogen atmosphere undergo declines in tensile properties and fatigueproperties due to hydrogen embrittlement. In particular, given thatfatigue failure is associated with 80% of failure accidents, there is aneed to elucidate the mechanisms for the hydrogen-induced decline infatigue properties and to pay very close attention to the fatigue designof hydrogen-related equipment. In light of the above, to ensure thesafety and reliability of hydrogen energy systems and infrastructure,there exists a desire for high-performance metal materials which do notexperience hydrogen-induced declines in tensile properties and fatigueproperties.

For example, at present, only the austenitic stainless steel SUS316L andthe aluminum alloy 6061-T6 have been approved for use as metal materialsexposed to hydrogen in fuel cell vehicles, with 6061-T6 being used asthe liner of hydrogen tank and SUS316L being used in pipes and varioustypes of valves and springs. Titanium alloys are used in the hydrogenstorage vessels disclosed in Patent Documents 1 and 2. Moreover,austenitic stainless steel is almost always assumed as the pipingmaterial to be used in the hydrogen pipelines currently being proposed.

-   Patent Document 1: Japanese Patent Application Laid-open No.    H10-38486-   Patent Document 2: Japanese Patent Application Laid-open No.    2007-298131

Today, the metal materials regarded as capable of withstanding use in ahydrogen environment, including the above-mentioned austenitic stainlesssteel, are all very expensive. Were these to be used in the constructionof a hydrogen infrastructure, the costs calculated based on the amountof such materials that would be needed in piping would be extremelyhigh. This has become an obstacle to the construction of a hydrogeninfrastructure. In addition, parts used in a hydrogen environment end upbeing expensive, which is a major factor holding back the popularizationof fuel cell vehicles and the like. By contrast, ferritic steels cost nomore than one-tenth as much as austenitic steel. However, when used in ahydrogen environment, their tensile properties and fatigue propertiesare far inferior to those of austenitic steels, making their use underconditions of exposure to hydrogen difficult at present.

DISCLOSURE OF THE INVENTION

This invention was conceived in light of the above circumstances. Theobject of the invention is to provide ferritic steels having tensileproperties and fatigue properties capable of withstanding use in ahydrogen environment, and a method of manufacturing the same.

The invention solves the above problems by the following means.

By adding one or more element selected from among vanadium (V), titanium(Ti) and niobium (Nb) so as to include, together with at least ferritegrains in the structure, a carbide or carbides of one or more elementselected from among V, Ti and Nb, the reduction of area and fatiguecrack propagation rate of ferritic steel in a hydrogen atmosphere areimproved. The advantages of the invention were confirmed in cases wherethe ferrite grains are small grains 1 μm or less in size, in cases wherethe ferrite grains are coarse grains from several micrometers to 20 μmin size, and in cases where the ferrite grains are coarse grains fromseveral micrometers to 60 μm in size.

The one or more element selected from among V, Ti and Nb is added in anamount which is preferably at least the amount required to fix allcarbon (C) in the structure as the carbide or carbides thereof. That is,an amount sufficient to fix all the carbon in the structure as vanadiumcarbide, titanium carbide, niobium carbide, or two or more of thesecarbides. The amount C* of carbon which can be fixed as the carbides VC,TiC and NbC having stoichiometric compositions may be obtained from thefollowing formula, wherein V^(C), Ti^(C) and Nb^(C) represent theamounts of the respective elements which bond with carbon.

$\begin{matrix}\begin{matrix}{C^{*} = {{\left( \frac{12.01}{50.94} \right)V^{C}} + {\left( \frac{12.01}{47.86} \right){Ti}^{C}} + {\left( \frac{12.01}{92.90} \right){Nb}^{C}}}} \\{= {{\left( \frac{1}{4.24} \right)V^{C}} + {\left( \frac{1}{3.99} \right){Ti}^{C}} + {\left( \frac{1}{7.74} \right){Nb}^{C}}}}\end{matrix} & (1)\end{matrix}$

Here, the units of C*, V^(C), Ti^(C) and Nb^(C) are mass %. The atomicweights of C, V, Ti and Nb were taken to be, respectively, 12.01, 50.94,47.86 and 92.90.

In order to fix all the carbon included in the structure, the followingmust hold

C<C*  (2)

(the units of C here are mass %).

Therefore, the amounts of addition for the respective elements are

V=V^(C),Ti=Ti^(C),Nb=Nb^(C)  (3).

Here, the amounts of V, Ti and Nb are in units of mass %.

In cases where vanadium has been added, a similar improvement in thereduction of area performance can be confirmed even without reaching theamount required to fix all the carbon (FIG. 14). The experimentalexamples shown below are examples in which only one element selectedfrom among V, Ti and Nb has been added. However, it is of courseacceptable for two selected from among these elements, or for all three,to be added.

The invention has the effect of improving the tensile properties andfatigue properties of ferritic steels in a hydrogen environment, andenabling them to withstand use under circumstances involving exposure tohydrogen. This makes it possible to markedly reduce the expensesrequired for building a hydrogen infrastructure. Moreover, the inventionalso makes it possible to greatly reduce production costs for parts usedin a hydrogen environment, such as hydrogen tank liners, pipelines andvarious types of valves and springs used in fuel cell vehicles, thusmaking it possible to provide fuel cell vehicles at lower prices. Inaddition, the invention makes it possible to hold down considerablyconstruction costs for hydrogen pipelines.

At present, SUS316L is used as the material in the pipelines throughwhich high-pressure hydrogen gas flows at hydrogen stations for 701 Mpa.SUS316L lines manufactured in accordance with the High-Pressure GasSafety Act have the following dimensions: in ½ inch pipe, an outsidediameter of 12.7 mm and an inside diameter of 3.1 mm; in ⅜ inch pipe, anoutside diameter of 10 mm and an inside diameter of 2.1 mm. In addition,hydrogen gas filling nozzles have an inside diameter of 1.6 mm. Becauseof such a small inside diameter, pressure loss sometimes occurs, causingthe flow rate during filling to become only a fraction of the initialdesign value. It is possible to improve the fill rate to some degree byusing SUS316, but at a very high cost. By using the ferritic steel ofthe invention as the pipe material, it is possible to construct ahydrogen station at a much lower cost than at present.

In recent years, SGP and STPG370 carbon steel pipes have beeninvestigated as candidate materials for hydrogen pipelines. However,from the standpoint of properties, the toughness decreases due to thepresence of pearlite in the carbon steel. Moreover, a problem withpearlite is that it becomes a hydrogen trapping site. Therefore,pearlite-free ferritic steel, owing to the improved tensile propertiesand fatigue properties in a hydrogen environment, is outstanding as acandidate material for hydrogen pipelines.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 shows photographs of the structure of S45C, FIG. 1( a) being atransverse section, and FIG. 1( b) being a longitudinal section.

FIG. 2 is a photograph of the structure of the fine-grained materialTi02-II (with 0.25 mass % Ti addition).

FIG. 3 is a photograph of the structure of the fine-grained materialV02-II (with 0.27 mass % Ti addition).

FIG. 4 is a photograph of the structure of the coarse-grained materialV02-I (with 0.2 mass % V addition) which was annealed at 600° C. for 1hour.

FIG. 5 is a photograph of the structure of the coarse-grained materialV04-I (with 0.4 mass % V addition) which was annealed at 600° C. for 1hour.

FIG. 6 is a photograph of the structure of the coarse-grained materialof Nb05-I (with 0.53 mass % Nb addition) which was annealed at 600° C.for 1 hour.

FIG. 7 is a photograph of the structure of the coarse-grained materialTi03-I (with additions of 0.3 mass % Ti and 50 mass ppm boron (B)) whichwas annealed at 600° C. for 1 hour.

FIG. 8 shows photographs of the structure of the coarse-grained materialV02-II (with 0.27 mass % V addition) which was annealed at 700° C. for 1hour, FIG. 8( a) being a transverse section and FIG. 8( b) being alongitudinal image.

FIG. 9 illustrates the relationship between the amount of absorbedhydrogen content and immersion time in a test specimen in the form of around bar of 8 mm diameter, FIG. 9( a) showing the relationship when thespecimen is made of S45C, and FIG. 9( b) showing the relationship whenthe specimen is made of the comparison base steel (fine-grainedmaterial) or Ti02-II (fine-grained material).

FIG. 10 shows test pieces used in fatigue life tests, FIG. 10( a) beinga smooth specimen, and FIG. 10( b) being a notched specimen.

FIG. 11 illustrates methods for reducing the stress intensity factorrange ΔK in a fatigue crack propagation test, FIG. 11( a) being a methodfor holding the stress ratio R constant and decreasing both the maximumload (Pmax) and the minimum load (Pmin), and FIG. 11( b) being a methodfor holding the maximum load (Pmax) constant and increasing the minimumload (Pmin) as the crack propagates.

FIG. 12 shows test pieces used in fatigue crack propagation tests, FIG.12( a) being a compact tension (CT) test piece, and FIG. 12( b) being aplate-type bending specimen.

FIG. 13 is a fatigue crack propagation test apparatus which uses aplate-type bending specimen.

FIG. 14 is a graph showing the relationship between the relativereduction of area φH/φ and the amount of absorbed hydrogen.

FIG. 15 is a graph showing the fatigue life properties of an S45C smoothspecimen.

FIG. 16 is a graph showing the fatigue life properties of an S45Cnotched specimen.

FIG. 17 is a graph showing the fatigue life properties of the comparisonbase steel (fine-grained material).

FIG. 18 is a graph showing the fatigue life properties of Ti02-II(fine-grained material).

FIG. 19 is a graph showing the fatigue life properties of V02-II(fine-grained material).

FIG. 20 is a graph showing the fatigue life properties of Nb04-II(fine-grained material).

FIG. 21 is a graph showing the fatigue life properties of V005(fine-grained material).

FIG. 22 is a graph showing the fatigue life properties ofV007-Nb01-Ti007 (fine-grained material).

FIG. 23 is a graph showing the fatigue crack propagation properties ofS45C when the stress ratio R was set to 0.1.

FIG. 24 is a graph showing the fatigue crack propagation properties ofS45C when the stress ratio R was set to 0.5.

FIG. 25 is a graph showing the fatigue crack propagation properties ofS45C when the stress ratio R was varied.

FIG. 26 is a graph showing the fatigue crack propagation properties atthe various stress ratios in FIGS. 23 to 25.

FIG. 27 is a graph showing the fatigue crack propagation properties ofV02-I (fine-grained material).

FIG. 28 is a graph showing the fatigue crack propagation properties ofV02-II (coarse-grained material: annealed at 700° C. for 1 hour).

FIG. 29 is a graph showing the fatigue crack propagation properties ofV005 (fine-grained material).

FIG. 30 is a graph showing the fatigue crack propagation properties ofV007-Nb01-Ti007 (fine-grained material).

FIG. 31 is a graph showing the relationship between the fatigue relativecrack propagation rate and the cycle speed for hydrogen-charged materialand uncharged material at R=0.5 and ΔK=10 MPa·m^(1/2).

FIG. 32 is a graph showing the relationship between the relative fatiguelife and the cycle speed for uncharged material and hydrogen-chargedmaterial at σa=350 MPa.

BEST MODE FOR CARRYING OUT THE INVENTION

In the invention, it was discovered that when ferritic steel to which atrace amount of at least one element selected from among vanadium (V),titanium (Ti) and niobium (Nb) has been added is hydrogen-charged thensubjected to tensile testing and fatigue testing, considerableimprovements with regard to the effects of hydrogen on the tensileproperties and the fatigue properties are achieved compared withconventional materials. Preferred modes for carrying out the inventionare described below in detail.

1. Test Materials

The chemical ingredients in the test materials are shown in Tables 1 to4. The balance in all of the materials was iron (Fe) and inadvertentimpurities. Table 1 shows the chemical ingredients in the common carbonsteel S45C for machine structural use which is used here for the sake ofcomparison.

TABLE 1 Chemical ingredients (mass %) in carbon steel S45C for machinestructural use C Si Mn P S Cu Ni Cr S45C 0.47 0.18 0.63 0.014 0.003 0.110.1 0.08

Table 2 shows the chemical ingredients in the comparison base steel.

TABLE 2 Chemical ingredients (mass %) in comparison base steel C Si MnCu Al O O—I 0.022 0.28 1.28 0.001 0.0009 0.002

As shown in Table 3 (Series I) and Table 4 (Series II), the materials ofthe invention are ferritic steels in which the base steel is0.05C-0.30Si-1.5Mn and to which a trace amount of at least one elementselected from among V, Ti and Nb has been added. Chemical analysis ofall the materials was carried out by inductively coupled plasma emissionspectroscopy. Here, the amounts of V, Ti and Nb addition are determinedbased on above formulas (1) to (3). When the carbon content is 0.05 mass%, the amounts of these respective elements needed to fix the carbon are0.212 mass % of V, 0.199 mass % of Ti, and 0.387 mass % of Nb. Thesevalues indicate the amount required to fix all the carbon when V, Ti orNb is added alone. As shown in Table 3, the amount of V addition inV02-I is 0.2 mass %, the amount of V addition in V04-I is 0.4 mass %,the amount of Ti addition in Ti03-I is 0.3 mass %, and the amount of Nbaddition in Nb05-I is 0.53 mass %. In V04-I, Ti03-I and Nb05-I, theamount of these respective elements suffices as the amount required tofix all the carbon. However, in V02-I, the amount of vanadium is lowerthan the amount required to fix all the carbon. As shown in Table 4, theamount of Ti addition in Ti02-II is 0.25 mass %, the amount of Vaddition in V02-II is 0.27 mass %, and the amount of Nb addition inNb04-II is 0.45 mass %. In each of these materials, the amount of theseelements suffices as the amount required to fix all the carbon. Thus, inthe Series I materials V02-I and V04-I shown in Table 3, the V additionsare respectively 0.2 mass % and 0.4 mass %; and in the Series IImaterial V02-II shown in Table 4, the V addition is 0.27 mass %. Hence,materials were prepared which contained amounts of these elements thatranged from less than to about twice as much as the amount required tofix all the carbon; that is, an addition of 0.212 mass %, as determinedbased on formulas (1) to (3).

TABLE 3 Chemical ingredients (mass %) in Series I B C Si Mn Ti V Nb(ppm) V02-I 0.05 0.30 1.5 0.2 V04-I 0.05 0.30 1.5 0.4 Nb05-I 0.05 0.301.5 0.53 Ti03-I 0.05 0.30 1.5 0.3 50

TABLE 4 Chemical ingredients (mass %) in Series II C Si Mn P S Ti V NbAl O Ti02-II 0.05 0.30 1.51 0.001 0.001 0.25 0.006 0.002 V02-II 0.050.31 1.51 0.002 0.001 0.27 0.005 0.003 Nb04-II 0.05 0.31 1.52 0.0010.002 0.45 0.010 0.003

Table 5 shows the heat treatment conditions and the thermomechanicaltreatment conditions for the test materials. As shown in Table 5(a),S45C used in the experiments below was obtained by annealing (heated at845° C. for 30 minutes, then allowed to cool in air), followed byquenching (so-called water quenching, which entails heating at 845° C.for 30 minutes followed by cooling in water), then tempering (heated at550° C. for 60 minutes, then allowed to cool in air).

TABLE 5 (a) Heat treatment conditions for S45C Annealing QuenchingTempering 30 minutes at 845° C., 30 minutes at 845° C., 60 minutes at550° C., cooled in air cooled in water cooled in air (b)Thermomechanical treatment conditions for fine-grained material ForgingRolling 60 minutes at 1170° C., 560° C., 95% rolling reduction, cooledin air cooled in water (c) Thermomechanical treatment conditions forcoarse-grained material Forging Rolling Annealing 60 minutes at 560° C.,95% rolling 60 minutes at 600° C. or 60 1170° C., reduction, minutes at700° C., cooled in air cooled in water cooled in air

In addition, base material as a control and various Series I and SeriesII ferritic steels subjected to the thermomechanical treatment shown inTable 5(b) were also prepared. That is, treatment entailed 60 minutes offorging at 1170° C., followed in turn by cooling in air, rolling at 560°C. and a rolling reduction of 95%, and cooling in water to form afine-grained structure. Ferritic steels subjected to this treatment arereferred to herein as “fine-grained materials.” In addition tofine-grained materials, various Series I and Series II ferritic steelssubjected to the thermomechanical treatment shown in Table 5(c) wereprepared. This treatment entailed carrying out the thermomechanicaltreatment in Table 5(b) and additionally carrying out 60 minutes ofannealing at 600° C. or 700° C. so as to obtain a grain size which isabout the same as that in conventional materials. Ferritic steelssubjected to this treatment are referred to herein as “coarse-grainedmaterials.”

The inventors also prepared two test materials having reduced additionsof V, Ti and Nb. The chemical ingredients of those test materials areshown in Table 6. In both materials, the balance was iron (Fe) andinadvertent impurities. In the material V005 shown in Table 6, 0.05 mass% of V has been added. This is substantially the amount required to fixall carbon (C). In the material V007-Nb01-Ti007, 0.07 mass % of V, 0.13mass % of Nb and 0.07 mass % of Ti have been added. That is, V has beenadded in about one-third the amount required to fix all the carbon withV alone, Nb has been added in about one-third the amount required to fixall the carbon with Nb alone, and Ti has been added in about one-thirdthe amount required to fix all the carbon with Ti alone. Collectively,these amounts of addition are approximately the same as the amountrequired to fix all the carbon. Fine-grained materials obtained bysubjecting V005 and V007-Nb01-Ti007 to the thermomechanical treatment inTable 5(b) were prepared.

TABLE 6 Chemical ingredients (mass %) in fine-grained material havingreduced additions C Si Mn Ti V Nb V005 0.05 0.30 1.5 0.05V007—Nb01—Ti007 0.05 0.30 1.5 0.07 0.07 0.13

FIGS. 1 to 4 show photographs of the microstructures. FIG. 1 showsoptical micrographs of S45C subjected to the heat treatment shown inTable 5(a); the structure is tempered martensite. FIG. 1( a) is atransverse section, and FIG. 1( b) is a longitudinal section. FIGS. 2and 3 are photographs of the structure of fine-grained materials, andwere obtained by preparation of a thin-film of the material andobservation with a transmission electron microscope (Hitachi, Ltd.).FIG. 2 is a photograph of the structure of the fine-grained materialTi02-II (with 0.25 mass % of Ti addition) shown in Table 4, and FIG. 3is similarly a photograph of the structure of the fine-grained materialV02-II (with 0.27 mass % of V addition) shown in Table 4. It waspossible to confirm from these photographs that the structure as a wholeis a fine ferrite grain structure, and that the ferrite grains of whichthe structure is primarily composed are fine grains having a size of 1μm or less.

FIGS. 4 to 7 are photographs of the structure of coarse-grainedmaterials obtained with one hour of annealing at 600° C., and FIG. 8shows photographs of the structure of a coarse-grained material obtainedwith one hour of annealing at 700° C. Both were examined under anoptical microscope after being corroded with an ordinary Nital solution(3 vol % nitric acid+ethanol). FIGS. 4, 5, 6 and 7 are respectivelyphotographs of the structures of the materials V02-I (with 0.2 mass % Vaddition), V04-I (with 0.4 mass % V addition), Nb05-I (with 0.53 mass %Nb addition) and Ti03-I (with 0.3 mass % Ti and 50 mass ppm boron (B)additions) shown in Table 3. From the photographs in FIGS. 4 to 7, itwas possible to confirm that the structure overall was a relativelycoarse ferrite grain structure, and that the ferrite grains of which thestructure is primarily composed are coarse grains having a size of fromseveral micrometers to 20 μm. FIG. 8 shows photographs of the structureof the material V02-II (with 0.27 mass % V addition) shown in Table 4,FIG. 8( a) being a transverse section, and FIG. 8( b) being alongitudinal section. It was possible to confirm from the photographs inFIG. 8 that the structure as a whole was a coarse ferrite grainstructure, and that the ferrite grains of which the structure isprimarily composed are coarse grains having a size of from severalmicrometers to 60 μm.

2. Hydrogen Charging Method

An immersion charging method was used to hydrogen charge the specimens.The hydrogen charging conditions used were in general accordance withthe proposed measures being studied for standardization by the Iron andSteel Institute of Japan and the Japan Society of Spring Engineers. Thatis, hydrogen charging was carried out by immersion in an aqueoussolution containing 20 mass % of ammonium thiocyanate. The temperatureof the aqueous solution was held at 40° C., and the charging time was 48hours. FIG. 9 shows an example of the relationship between the amount ofabsorbed hydrogen and the immersion time in round bars of 8 mm diameteras the specimens. FIG. 9( a) shows the relationship when the specimen ismade of S45C, and FIG. 9( b) shows the relationship when the specimen ismade of a fine-grained comparison base steel or of the fine-grainedmaterial Ti02-II. Because the amount of absorbed hydrogen in FIG. 9reaches saturation in about 15 hours, the charging time in thisinvention was set to 48 hours.

3. Tensile Test Method

The tensile test was carried out using an autoclave having a maximumcapacity of 100 kN (Shimadzu Corporation) and based on JIS B7721. Thetest rate was 0.5 mm/min. The specimens were No. JIS14A bars having adiameter of 5 mm and a gauge length L of 25 mm.

4. Fatigue Test Methods 4-1. Fatigue Life Test

The fatigue life test was carried out using a hydraulic servo-typetension-compression fatigue tester (Shimadzu Corporation) having amaximum capacity of 50 kN, and under sinusoidal uniaxial loading. Thestress ratio R (minimum stress/maximum stress: σmin/σmax) was −1. Thetest was carried out only at a cycle speed of 30 Hz on the unchargedmaterial, and at three cycle speeds—0.2 Hz, 2 Hz and 30 Hz—onhydrogen-charged materials. However, in the case of S45C and thefine-grained comparison base steel, because the fatigue life had astrong cycle speed dependence, tests at 0.02 Hz were also carried out.The fatigue tests carried out were primarily low life-side (highstress-side) tests in which hydrogen release from the specimen was low.The specimens used in the fatigue life tests are shown in FIG. 10. Theseare specimens with shapes that are commonly used in fatigue life tests.Use was made of both the smooth specimen shown in FIG. 10( a) and thenotched specimen shown in FIG. 10( b). The smooth specimen was a roundbar having parallel sections with a diameter of 8 mm. In the notchedspecimen, an annular V-shaped notch with a depth of 1.5 mm wasintroduced at the center of a smooth specimen. The stress intensityfactor Kt was 3.7. Finishing treatment at the surface in the test regionof the specimen consisted of axial polishing with No. 600 sandpaper (JISR6252).

4-2. Fatigue Crack Propagation Test

To further clarify the effects of hydrogen on the fatigue properties, afatigue crack propagation test was performed on some of the materials.The fatigue crack propagation test was carried out using a hydraulicservo-type tension compression fatigue test having a maximum capacity of10 kN, and under sinusoidal loading at a cycle speed of 30 Hz. The twomethods shown in FIG. 11 were used as the methods for reducing thestress intensity factor range ΔK. One was a method in which, as shown inFIG. 11( a), both the maximum load (Pmax) and the minimum load (Pmin)are reduced while keeping the stress ratio R constant (ΔK reducing testat constant R=0.1, or ΔK reducing test at constant R=0.5). This methodis commonly used. The other is a method in which, as shown in FIG. 11(b), the maximum load (Pmax) is kept constant and the minimum load (Pmin)is raised as cracking proceeds (ΔK reducing test at constant PMax). TheΔK reduction ratio was set at dΔK/da=−2 Gpa·m^(1/2) in both tests.

In the ΔK reducing tests at constant Pmax, when ΔK decreases as thecrack progresses, the stress ratio R gradually rises. Hence, by settingthe initial test conditions to R≧0.5 and ΔK≧7 MPa·m^(1/2), complex crackclosing behavior can be avoided until the fatigue crack thresholdΔK_(th) is reached. In this way, the influence of hydrogen on fatiguecrack propagation can be clearly understood. To clarify the cycle speeddependency, 0.2 Hz and 2 Hz tests were also carried out on Series I andSeries II materials having trace element additions. In these cases, thetests were carried out near ΔK=10 MPa·m^(1/2) and at R=0.5 and aconstant load amplitude ΔP. The crack lengths were measured at intervalsof 0.2 mm or 0.1 mm using both the alternating current potential methodand the compliance method (a method of measuring the crack length fromthe output of a strain gauge attached to the back of the specimen). A 1mm pre-crack was introduced at R=0.1 and ΔK=15 MPa·m^(1/2).

The specimen had a shape commonly used in fatigue crack propagationtests. Because the Series I and Series II stock was in the form of 17 mmsquare bar, the plate-like bending specimens having a width of 12 mm anda plate thickness of 10 mm shown in FIG. 12( b) were used. In caseswhere plate-like bending specimens were used, crack propagation testswere carried out using the test apparatus shown in FIG. 13. On the otherhand, because the S45C stock was large, the compact tension (CT)specimens having a plate width of 35 mm and a plate thickness of 6 mmshown in FIG. 12( a) were used. In this case, the load was applied bypin connection. Some plate-type bending specimen tests were also carriedout on S45C, from which there was confirmed to be no difference with theresults obtained using CT specimens.

4-3. Hydrogen Analysis Method

Following completion of the fatigue tests, samples were immediately cutfrom the test specimens, and the amount of absorbed hydrogen wasmeasured with a gas chromatograph-type thermal differential analyzer(TDA). The ramp-up rate was set at 100° C./h up to an ultimatetemperature of 600° C., and the cumulative amount of hydrogen releasedup to 500° C. was treated as the amount of absorbed hydrogen.

5. Test Results 5-1. Tensile Properties

Table 7(a) shows the tensile test results for the S45C unchargedmaterial, and Table 7(b) shows the tensile test results for the S45Chydrogen-charged material. Table 8(a) shows the test results for SeriesI (fine-grained) uncharged materials, Table 8(b) shows the test resultsfor Series I (fine-grained) hydrogen-charged materials, and Table 8(c)shows the test results for Series I (fine-grained) hydrogen-chargedmaterials which were 3% pre-strained. Table 9(a) shows the test resultsfor Series II (fine-grained) uncharged materials, and Table 9(b) showsthe test results for Series II (fine-grained) hydrogen-chargedmaterials. In addition, Table 10(a) shows the test results for Series I(coarse-grained: annealed at 600° C. for 1 hour) uncharged materials,and Table 10(b) shows the test results for Series I (coarse-grainedmaterial: annealed at 600° C. for 1 hour) hydrogen-charged materials.

TABLE 7 (a) S45C Uncharged material 0.2% offset yield Tensile ReductionTest strength strength Elongation of area piece (N/mm²) (N/mm²) (%) (%)S45C 778 906 20 62

TABLE 8 (b) S45C Hydrogen-charged material 0.2% offset yield TensileReduction Amount of Test strength strength Elongation of area hydrogenpiece (N/mm²) (N/mm²) (%) (%) (mass ppm) S45C 808 921 18 50 1

TABLE 8 (a) Series I (fine-grained) uncharged material 0.2% offset yieldTensile Reduction Test strength strength Elongation of area piece(N/mm²) (N/mm²) (%) (%) V02-I 1003 1007 10 69 V04-I 1017 1060 18 74Nb05-I 682 860 14 76 Ti03-I 638 786 18 74 (b) Series I (fine-grained)hydrogen-charged material 0.2% offset yield Tensile Reduction Amount ofTest strength strength Elongation of area hydrogen piece (N/mm²) (N/mm²)(%) (%) (mass ppm) V02-I 982 991 13.5 74 1.4 V04-I 1046 1071 14.9 70 3.4Nb05-I 692 869 12.4 58 4.4 Ti03-I 676 786 12.8 62 3.5 (c) Series I(fine-grained) hydrogen-charged material that was 3% pre-strained 0.2%offset yield Tensile Reduction Amount of Test strength strengthElongation of area hydrogen piece (N/mm²) (N/mm²) (%) (%) (mass ppm)V02-I 964 966 12.6 75 1.2 V04-I 1078 1085 14.2 71 4.2 Nb05-I 832 888 9.756 4.2 Ti03-I 800 807 9.6 58 3.5

TABLE 9 (a) Series II (fine-grained) uncharged material 0.2% offsetyield Tensile Reduction Test strength strength Elongation of area piece(N/mm²) (N/mm²) (%) (%) Ti02-II 671 790 20 79 V02-II 946 937 20 76Nb04-II 772 883 18 74 (b) Series II (fine-grained) hydrogen-chargedmaterial 0.2% offset yield Tensile Reduction Amount of Test strengthstrength Elongation of area hydrogen piece (N/mm²) (N/mm²) (%) (%) (massppm) Ti02-II 683 789 18 69 5.37 V02-II 952 946 18 76 2.38 Nb04-II 778885 17 65 4.31

TABLE 10 (a) Series I (coarse-grained for 1 hour at 600° C.) unchargedmaterial 0.2% offset yield Tensile Reduction Test strength strengthElongation of area piece (N/mm²) (N/mm²) (%) (%) V02-I 760 770 26 81V04-I 808 861 22 77 Nb05-I 683 734 25 77 Ti03-I 455 546 33 78 (b) SeriesI (coarse-grained for 1 hour at 600° C.) hydrogen-charged material 0.2%offset yield Tensile Reduction Amount of Test strength strengthElongation of area hydrogen piece (N/mm²) (N/mm²) (%) (%) (mass ppm)V02-I 731 746 28 78 1.2 V04-I 838 875 20 74 2.9 Nb05-I 635 737 19 69 2.4Ti03-I 491 553 31 77 1.3

The amounts of hydrogen shown in these tables are the amounts ofabsorbed hydrogen in the specimens, as measured following tensiletesting. Because the tensile test time is only about 15 minutes long,hydrogen release during the test is low. Here, in the test results shownin Table 7, even though hydrogen charging was carried out on S45C,decreases in the 0.2% offset yield strength and the tensile strength arenot observed. On the other hand, decreases in the elongation andreduction of area are observed; in particular, the decrease in thereduction of area was pronounced. The reduction of area is generallyused to assess the influence of hydrogen on the tensile properties. Fromthe results in Table 7, it was possible to confirm a decrease in thereduction of area due to the influence of hydrogen in S45C having noadditions of V, Nb or Ti.

In the test results shown in Tables 8 to 10, on comparing unchargedmaterials with hydrogen-charged materials, there were no largedifferences in the 0.2% offset yield strength (stress at the time of0.2% plastic deformation) and the tensile strength in any of thespecimens. This was the same as for S45C. However, a characteristic ofSeries I and Series II materials is that the reduction of area inhydrogen-charged materials exhibits no decrease or decreases onlyslightly if at all compared with that in uncharged materials. That is,by adding any one of the elements V, Nb or Ti to ferritic steel, theinfluence of hydrogen on the tensile properties can be decreased.

FIG. 14 shows the relationship between the relative reduction of areaφH/φ and the amount of absorbed hydrogen. φH is the reduction of areafor a hydrogen-charged material, and φ is the reduction of area for anuncharged material. Results for the carbon steel STPG370, which isregarded as a candidate material for gas pipelines, have also beenincluded in the graph for the sake of comparison. In S45C and STPG370,the relative reduction of area decreases sharply as the amount ofabsorbed hydrogen C_(H) rises. However, in Series I (fine-grainedmaterial, coarse-grained material) and Series II (fine-grained material)ferritic steels with trace element (V, Nb, Ti) additions, the decreasein the relative reduction of area is gradual and the reduction of areaperformance is greatly improved. It was confirmed from these resultsthat the addition of a trace amount of V, Ti or Nb has a desirableeffect on recovery from a decline in the reduction of area (ductility).

Here, on comparing two materials having 0.20 mass % additions of V,namely V02-I (fine-grained material) and V02-I (coarse-grainedmaterial), with V02-II (fine-grained material) having a 0.27 mass %addition of V and V04-I (fine-grained material) and V04-I(coarse-grained material) having 0.40 mass % additions of V, it can beconfirmed that there are no clear differences in the relative reductionof area. Hence, it is not necessarily the case that all the carbon mustbe fixed in order to improve the reduction of area and make ferriticsteel capable of withstanding use in a hydrogen atmosphere. While addingsufficient additive to fix all the carbon is of course acceptable,because V, Ti and Nb are all expensive, when taking cost into account,it is desirable to minimize the amounts in which these are used.

5-2. Fatigue Properties

FIG. 15 shows the relationship between the S—N properties of a smoothspecimen of S45C, i.e., the stress amplitude σa (stress amplitudeΔσ=(maximum stress σmax−minimum stress σmin)×½), and the failure life.In the diagram, the residual amount of hydrogen (mass ppm) measured in asample cut from the fatigue failure specimen is also shown. Because thetesting time extends to 66 hours at the lowest cycle speed 0.02 Hz, theresidual amount of hydrogen in this case was only 0.42 mass ppm.However, in high-speed tests at 0.2 Hz and 2 Hz, the residual amounts ofhydrogen were respectively 0.71 mass ppm and 1.03 mass ppm, indicatingthat much hydrogen remained. From FIG. 15, regardless of the cyclespeed, the failure life of the hydrogen-charged material agreedsubstantially with the results for the uncharged material, and so noinfluence by hydrogen was observable.

FIG. 16 shows the S—N properties of S45C notched specimens. In this caseas well, with the exception of the lowest cycle speed 0.02 Hz, muchhydrogen remained. When the fatigue lives of hydrogen-charged materialsat test rates of 0.2 Hz, 2 Hz and 30 Hz were compared with the resultsfor the respective uncharged materials, the fatigue life of thehydrogen-charged material became progressively shorter at lower cyclespeeds. In this diagram, the fatigue life at a cycle speed of 30 Hz wasfrom about 15,000 to about 35,000 cycles, the fatigue life at 2 Hz wasabout 6,000 cycles, and the fatigue life at 0.2 Hz was about 2,000cycles. The fatigue lives do not shorten from 0.2 Hz to 0.02 Hz,indicating that saturation occurs once the fatigue life decreases toabout 6 to 13% of the fatigue life at 30 Hz.

Summarizing the results in FIGS. 15 and 16, in ordinary materials suchas S45C, the S—N properties (fatigue crack generation) obtained with asmooth specimen do not incur a hydrogen influence, but the S—Nproperties obtained with a notched specimen (fatigue crack propagation)do incur a hydrogen influence. Hence, the S—N properties obtained with anotched specimen were examined in fine-grained materials. As shown inFIG. 17, in the comparison base steel, the fatigue life at a cycle speedof 30 Hz was about 30,000 to 100,000 cycles, the fatigue life at 2 Hzwas about 12,000 cycles, and the fatigue life at 0.2 Hz was about 6,000cycles. The fatigue life did not shorten from 0.2 Hz to 0.02 Hz,indicating that saturation occurs once the fatigue life decreases toabout 6 to 20% of the life at 30 Hz.

As shown in FIG. 18, in the Series II material Ti02-II (fine-grainedmaterial) with Ti addition, the fatigue life at a cycle speed of 30 Hzis from about 80,000 to about 300,000 cycles, the fatigue life at 2 Hzis about 50,000 cycles, and the fatigue life at 0.2 Hz is about 45,000cycles. Here, the fatigue life decreases to about 15 to 56% of thefatigue life at 30 Hz, although the degree of decrease is more gradualthan in the above-described S45C and comparison base steel. That is,Ti02-II has a fatigue life (the ratio of the life at 0.2 Hz to the lifeat 30 Hz) which is from 2.5 to 4.3 times that of S45C, and from 2.5 to2.8 times that of the comparison base steel.

As shown in FIG. 19, in the Series II material V02-II (fine-grainedmaterial) with V addition, the fatigue life at a cycle speed of 30 Hz isabout 30,000 to 90,000 cycles, the fatigue life at 2 Hz is about 10,000cycles, and the fatigue life at 0.2 Hz is about 8,000 cycles. Here, thefatigue life decreases to about 9 to 27% of the fatigue life at 30 Hz.That is, V02-II has a fatigue life (the ratio of the life at 0.2 Hz tothe life at 30 Hz) which is from 1.5 to 2.1 times that of S45C, andabout 1.5 times that of the comparison base steel. As shown in FIG. 20,in the Series II material Nb04-II (fine-grained material) with Nbaddition, the fatigue life at a cycle speed of 30 Hz is about 90,000 to350,000 cycles, the fatigue life at 2 Hz is about 40,000 cycles, and thefatigue life at 0.2 Hz is about 50,000 cycles. Here, the fatigue lifedecreases to about 12 to 56% of the life at 30 Hz. That is, Nb02-II hasa fatigue life (the ratio of the life at 0.2 Hz to the life at 30 Hz)which is from 2 to 4.3 times that of S45C, and about 2 to 2.8 times thatof the comparison base steel. These results indicate that, infine-grained materials with trace additions of Ti, V or Nb, thehydrogen-induced fatigue crack propagation properties improve.

As shown in FIG. 21, with regard to V005 (fine-grained material), thefatigue life at a cycle speed of 30 Hz is about 20,000 cycles, thefatigue life at 2 Hz is about 8,000 cycles, and the fatigue life at 0.2Hz is about 6,000 cycles. Here, the fatigue life decreases to about 30%of the life at 30 Hz. That is, V0005 has a fatigue life (the ratio ofthe life at 0.2 Hz to the life at 30 Hz) which is from 2.3 to 5 timesthat of S45C, and from about 1.5 to about 5 times that of the comparisonbase steel. As indicated by these results, even when V is added in aboutone-fourth the amount required to fix all the carbon, thehydrogen-induced fatigue crack propagation properties can be improved.

As shown in FIG. 22, with regard to V007-Nb01-Ti007 (fine-grainedmaterial), the fatigue life at a cycle speed of 30 Hz is about 30,000cycles, the fatigue life at 2 Hz is about 15,000 cycles, and the fatiguelife at 0.2 Hz is about 12,000 cycles. Here, the fatigue life decreasesto about 60% of the life at 30 Hz. That is, this material has a fatiguelife (the ratio of the life at 0.2 Hz to the life at 30 Hz) which isfrom 4.6 to 10 times that of S45C, and from about 3 to about 10 timesthat of the comparison base steel. As indicated by these results, whenall three elements V, Nb and Ti are added and the collective amount ofthese elements is about the same as the amount required to fix all thecarbon, extremely good fatigue crack propagation properties wereobtained.

Based on the above results, the influence of hydrogen on the fatiguecrack propagation properties were carefully studied. FIGS. 23 to 28 showthe fatigue crack propagation properties, i.e., the relationship betweenthe fatigue crack propagation rate da/dN (mm/cycle) and the stressintensity factor range ΔK (MPa·m^(1/2)). The fatigue crack propagationproperties of S45C are shown in FIGS. 23 to 26, those of the Series Imaterial V02-I (fine-grained material) are shown in FIG. 27, and thoseof the Series II material V02-II (coarse-grained material: annealed onehour at 700° C.) are shown in FIG. 28. Also, the fatigue crackpropagation properties of V005 (fine-grained material) are shown in FIG.29, and those of V007-Nb01-Ti007 (fine-grained material) are shown inFIG. 30. In each of these figures, the amount of absorbed hydrogen (massppm) measured after testing is shown below the fatigue crack propagationcurves for the hydrogen-charged materials. The testing time was set to30 hours or less in order to minimize hydrogen release during the test.Hence, the influence of hydrogen release during testing on the fatiguecrack propagation properties was small.

FIG. 23 shows fatigue crack propagation curves for uncharged materialand hydrogen-charged material when the stress ratio R is 0.1. Thedifference between the two curves can be clearly distinguished. Inparticular, when ΔK=7.0 MPa·m^(1/2), because the uncharged material hasa da/dN of about 9×10⁻⁸ (mm/cycle) and the hydrogen-charged material hasa da/dN of about 3×10⁻⁶ (mm/cycle), the crack propagation rate da/dN ofthe hydrogen-charged material can be seen to represent an up to 30-foldacceleration over that in uncharged material. Also, as shown in FIGS.24, 25 and 26, even in cases where differing stress ratios R were used,the fatigue crack propagation curves for uncharged material andhydrogen-charged material can be clearly distinguished; the crackpropagation rate da/dN of the hydrogen-charged materials was found torepresent an acceleration of at least 10-fold compared with unchargedmaterials.

With regard to the Series I material V02-I (fine-grained material) shownin FIG. 27, the fatigue crack propagation properties of the unchargedmaterial and the hydrogen-charged material are very similar to eachother, making it impossible to distinguish between the fatigue crackpropagation curves for both. That is, even in cases where hydrogencharging was carried out, substantially no acceleration of da/dN arose.Also, with regard as well to the Series II material V02-II(coarse-grained material: annealed one hour at 700° C.) shown in FIG.28, the fatigue cracking propagation properties of the unchargedmaterial and the hydrogen-charged material were very similar, making itimpossible to distinguish between the fatigue crack propagation curvesfor both. Hence, even when hydrogen charging was carried out,substantially no acceleration of da/dN arose.

FIG. 29 shows the fatigue crack propagation properties of V005(fine-grained material) in which the amount of V addition has beenreduced. When the stress ratio R is 0.1, the fatigue crack propagationcurves for uncharged material and hydrogen-charged material can be saidto generally coincide. On the other hand, when R=0.6 to 0.9, the twocurves separate at and above ΔK=5 MPa·m^(1/2), but below ΔK=5MPa·m^(1/2) the two curves generally coincide and substantially noacceleration of fatigue crack propagation due to hydrogen charging isobservable. Therefore, even with regard to the material V005, in whichthe amount of V addition was set at one-fourth of the amount required tofix all the carbon, it was possible to confirm effects due to Vaddition. However, at R=0.6 and above and at ΔK=5 MPa·m^(1/2) and above,such effects cease to be clear and so it may be regarded as preferableto add one or more element selected from among V, Nb and Ti in at leastone-fourth the amount required to fix all the carbon.

FIG. 30 shows the fatigue crack propagation properties ofV007-Nb01-Ti007 (fine-grained material). Regardless of the stress ratioR, the fatigue crack propagation curves for the uncharged material andthe hydrogen-charged material are very similar, with no separation beingapparent. From these results, it is clearly advantageous to add all ofthe elements V, Nb and Ti and to have the collective amount of these besubstantially the same as the amount required to fix all the carbon.

Based on the experimental results in FIGS. 23 to 28 and also the resultsof additional experiments that were carried out, FIG. 31 shows therelationship between the fatigue relative crack propagation rate forhydrogen-charged materials and uncharged materials between the cyclespeed f at R=0.5 and ΔK=10 MPa·m^(1/2). The ordinate represents thefatigue relative crack propagation rate (da/dN)_(H)/(da/dN). Here,(da/dN)_(H) is the fatigue crack propagation rate of thehydrogen-charged material, and da/dN is the fatigue crack propagationrate of the uncharged material. The abscissa represents the cycle speedf (Hz). The fatigue relative crack propagation rate of S45C (indicatedby the symbol ο in FIG. 31) is about 30 at a cycle speed f=0.2 Hz, about30 at f=2 Hz, and about 4 at f=30 Hz. With hydrogen charging, thefatigue crack propagation rate is accelerated up to 30-fold.

In the case of the comparison base steel (fine-grained material) (Δ inFIG. 31), the fatigue relative crack propagation rate is about 5 at thecycle speed f=0.2 Hz, about 4 at f=2 Hz, and about 3 at f=30 Hz.However, with regard to the fine-grained material V02-I V (┌ in FIG. 31)and the coarse-grained material V02-II (⋄ in FIG. 31), each of which hasa trace amount of V addition, in both of these cases, the fatiguerelative crack propagation rate is about 2 at the cycle speed f=0.2 Hz,about 2 at f=2 Hz, and about 1 at f=30 Hz, demonstrating that thehydrogen-induced acceleration of fatigue crack propagation rate can begreatly ameliorated due to a trace amount of V addition. Moreover, fromthe results for the fine-grained material V02-I (in FIG. 31), sufficientimprovement effects can be obtained, even with regard to the fatiguecrack propagation rate, by adding a smaller amount of V than the amountV^(C) required to fix the carbon. In other words, it is not necessarilyessential to fix all the carbon in order to improve the fatigue crackpropagation rate.

With regard to V005 (fine-grained material), as shown in FIG. 31, thefatigue relative crack propagation rate is about 3.5 at a cycle speedf=0.2 Hz, is about 3 at f=2 Hz, and is about 2.8 at f=30 Hz. Hence,compared with the comparison base steel (fine-grained material), it waspossible to ameliorate hydrogen-induced acceleration of the fatiguecrack propagation rate. However, the effects compared with thefine-grained material V02-I (┌ in FIG. 31) and the coarse-grainedmaterial V02-II (⋄ in FIG. 31) are limited and, as can be seen also bycomparison with other results presented in the same graph, when theamount of V addition is reduced even further from that in V005,advantageous effects cease to be observable relative to the comparisonbase steel (fine-grained material). Based on these results as well, itcan be said that the lower limit in the amount of addition for obtaininga fatigue crack propagation acceleration-ameliorating effect is aboutone-fourth of the amount required to fix all the carbon. With regard tothe fine-grained material V007-Nb01-Ti007, this exhibits propertiescomparable to or better than those of the fine-grained material V02-I (□n FIG. 31) and the coarse-grained material V02-II (⋄ in FIG. 31), and sofatigue crack propagation rate-ameliorating effects from adding all ofelements V, Nb and Ti and setting the collective amount of theseelements to about the same amount as that required to fix all the carbonwere observed.

FIG. 32 is a graph showing the relationship between the relative fatiguelife Nf/(Nf)_(H) at σa=350 MPa and the cycle speed f, based on the S—Ncurves for notched specimens (FIGS. 16 to 20). Here, Nf is the number ofcycles to failure of the uncharged material, and (Nf)_(H) is the numberof cycles to failure of the hydrogen-charged material. When specimenshaving a sharp notch (Kt=3.7) are used, the relative fatigue life ofS45C is about 10 at a cycle speed f=0.2 Hz, about 3.5 at f=2 Hz, andabout 1.2 at f=30 Hz. The relative fatigue life of the comparison basesteel is about 5 for a cycle speed f=0.2 Hz, about 3 for f=2 Hz, andabout 1.1 for f=30 Hz.

In the case of the fine-grained material Ti02-II with a trace amount ofTi addition, the relative fatigue life is about 1.2 at a cycle speedf=0.2 Hz, about 1.0 at f=2 Hz, and about 0.6 at f=30 Hz. In the case ofthe fine-grained material V02-II with a trace amount of V addition, therelative fatigue life is about 2.3 at a cycle speed f=0.2 Hz, about 1.9at f=2 Hz, and about 0.9 at f=30 Hz. Hence, the ratio Nf/(Nf)_(H) andthe ratio (da/dN)_(H)/(da)/(dN) shown in FIG. 31 substantially coincide.In the case of the fine-grained material Nb04-II with a trace amount ofNb addition, the relative fatigue life is about 1.3 at a cycle speedf=0.2 Hz, about 1.9 at f=2 Hz, and about 0.9 at f=30 Hz. From theseresults, not only in cases where a trace amount of V has been added, butalso in cases where a trace amount of Ti or Nb has been added, there canbe said to be an ameliorating (i.e., suppressing) effect on thehydrogen-induced acceleration of fatigue crack propagation.

With regard to the fine-grained material V005, as shown in FIG. 32,compared with the comparison base steel (fine-grained material), thishas a slight relative fatigue life-improving effect. Therefore, whentrying to improve the relative fatigue life, it may regarded asessential to set the amount of addition to at least about one-fourth theamount required to fix all the carbons. At the same time, thefine-grained material V007-Nb01-Ti007 exhibits properties comparable toor better than those of the fine-grained material V02-II (⋄ in FIG. 32),from which it was possible to confirm that adding all the elements V, Nband Ti and setting the collective amount of addition thereof to aboutthe same amount as that required to fix all the carbon has a relativefatigue life-improving effect.

INDUSTRIAL APPLICABILITY

The invention provides ferritic steels which are capable of being usedunder a hydrogen atmosphere. The ferritic steels of the invention can beemployed as structural materials in hydrogen energy systems such as fuelcell vehicles, and in hydrogen energy infrastructure such as hydrogenstations.

1. A hydrogen fatigue-resistant ferritic steel, comprising, with one ormore element selected from among vanadium (V), titanium (Ti) and niobium(Nb) being added, a carbide or carbides of one or more element selectedfrom among V, Ti and Nb in a structure composed primarily of ferritegrains, wherein the ferritic steel exhibits improvement in reduction ofarea and in a fatigue crack propagation rate in a hydrogen atmosphere.2. The hydrogen fatigue-resistant ferritic steel of claim 1, wherein thestructure is composed primarily of fine ferrite grains having a diameterof 1 μm or less.
 3. The hydrogen fatigue-resistant ferritic steel ofclaim 1, wherein the structure is composed primarily of coarse ferritegrains having a diameter of from several micrometers to 20 μm.
 4. Thehydrogen fatigue-resistant ferritic steel of claim 1, wherein thestructure is composed primarily of coarse ferrite grains having adiameter of from several micrometers to 60μ.
 5. The hydrogenfatigue-resistant ferritic steel of any one of claims 1 to 4, whereinthe one or more element selected from among V, Ti and Nb has been addedin at least the amount required to fix all carbon (C) in the structureas the carbide or carbides.
 6. The hydrogen fatigue-resistant ferriticsteel of any one of claims 1 to 4, comprising carbides of all theelements V, Ti and Nb, wherein the V, Ti and Nb have been added in acollective amount which is substantially the same as the amount requiredto fix all carbon (C) in the structure as the carbides.
 7. The hydrogenfatigue-resistant ferritic steel of any one of claims 1 to 4, whereinthe one or more element selected from among V, Ti and Nb has been addedin an amount which is less than, but at least one-fourth of, the amountrequired to fix all carbon (C) in the structure as the carbide orcarbides.
 8. The hydrogen fatigue-resistant ferritic steel of any one ofclaims 1 to 4, wherein, of the V, Ti and Nb, the only element that hasbeen used as an additive is V, and the V has been added in an amountwhich is less than the amount required to fix all carbon (C) in thestructure as a carbide of V.
 9. The hydrogen fatigue-resistant ferriticsteel of any one of claims 1 to 4, wherein, of the V, Ti and Nb, theonly element that has been used as an additives is V, and the V has beenadded in an amount which is less than, but at least one-fourth of, theamount required to fix all carbon (C) in the structure as a carbide ofV.
 10. A method of manufacturing a hydrogen fatigue-resistant ferriticsteel, the method comprising a step of adding one or more elementselected from among vanadium (V), titanium (Ti) and niobium (Nb) so asto include a carbide or carbides of one or more element selected fromamong V, Ti and Nb in a structure composed primarily of ferrite grains,and thereby improving the reduction of area and fatigue crackpropagation rate of ferritic steel in a hydrogen environment.
 11. Themethod of manufacturing a hydrogen fatigue-resistant ferritic steel ofclaim 10, wherein the one or more element selected from among V, Ti andNb is added in at least the amount required to fix all carbon (C) in thestructure as the carbide or carbides.
 12. The method of manufacturing ahydrogen fatigue-resistant ferritic steel of claim 10, wherein theferritic steel includes carbides of all the elements V, Ti and Nb, andV, Ti and Nb are added in a collective amount which is substantially thesame as the amount required to fix all carbon (C) in the structure asthe carbides.
 13. The method of manufacturing a hydrogenfatigue-resistant ferritic steel of claim 10, wherein the one or moreelement selected from among V, Ti and Nb is added in an amount which isless than, but at least one-fourth of, the amount required to fix allcarbon (C) in the structure as the carbide or carbides.
 14. The methodof manufacturing a hydrogen fatigue-resistant ferritic steel of claim10, wherein, of the V, Ti and Nb, the only element to be added is V,said V being added in an amount which is less than the amount requiredto fix all carbon (C) in the structure as a carbide of V.
 15. The methodof manufacturing a hydrogen fatigue-resistant ferritic steel of claim10, wherein, of the V, Ti and Nb, the only element to be added is V,said V being added in an amount which is less than, but at leastone-fourth of, the amount required to fix all carbon (C) in thestructure as a carbide of V.